As an image forming material comprising a positive working light sensitive layer to be solubilized by actinic light irradiation, an image forming material comprising a light sensitive layer containing an acid generating compound and an acid decomposable compound is known. That is, a light sensitive composition containing an acid generating compound and a water insoluble compound having a specific group capable of decomposed by an acid is disclosed in U.S. Pat. No. 3,779,779, a light sensitive composition containing an acid generating compound and a compound having an acetal or a ketal in the main chain is disclosed in Japanese Patent O.P.I. Publication No. 53-133429/1978, and a light sensitive composition containing an acid generating compound and a compound having a silylether group is disclosed in Japanese Patent O.P.I. Publication No. 65-37549/1985. These compositions have sensitivity in the ultraviolet range, which are capable of being alkali solubilized by imagewise ultraviolet ray exposure to provide non-image portions at exposed portions and to provide image portions at non-exposed portions. The imagewise ultraviolet ray exposure is generally carried out through a mask film by employing ultraviolet rays emitted from a light source such as a halogen lamp or a high pressure mercury lamp, or can be carried out by employing a short wavelength laser such as an argon laser or a helium-cadmium laser. However, these light sources are expensive, and troublesome in using due to its large size. Further, the above described light sensitive materials could not necessarily provide a satisfactory result in view of sensitivity.
In order to increase sensitivity of light sensitive materials, the present inventor proposed a positive working light sensitive material comprising a mixture of solvents different in viscosity in Japanese Patent O.P.I. Publication No. 8-320556, and a positive working light sensitive material comprising a light sensitive compound, its good solvent and its poor solvent in Japanese Patent O.P.I. Publication No. 8-328245. However, the imagewise exposure employing ultraviolet rays still has the problems as described above. A simple and less expensive exposure method has been eagerly sought.
A technique for forming an image by irradiation of infrared rays of a cheap and compact semiconductor laser is proposed. In U.S. Pat. No. 5,340,699 is disclosed an image forming material comprising a light sensitive layer containing an acid generating compound, a resol resin, a novolak resin and an infrared absorber, wherein a negative image is formed by a method comprising imagewise exposing the material to infrared rays, heating the exposed material before development, and then developing the heated material, or a positive image is formed by a method comprising imagewise exposing the material to infrared rays, and then developing the exposed material without heating. In Japanese Patent O.P.I. Publication No. 53-133429 is disclosed an image forming material comprising a light sensitive layer containing an acid generating compound, an acid decomposable compound and a blue dye (crystal violet). However, these techniques can not provide a good image, since laser light is irregularly reflected on the support surface, resulting in dot area fluctuation and poor image reproduction. Particularly when a presensitized planographic printing plate employing an aluminum plate as a support is exposed to a semi-conductor laser light to form an image, the laser light is reflected on the support surface to diffuse a heat energy, which causes dot gain and sensitivity lowering, resulting in serious problems in image forming.